The present invention relates to processes for producing modified composite particles based on organic polymers and fine organic solids, especially silicon oxides such as waterglass or silica sols, to the modified composite particles obtainable by the processes and to the use thereof, for example as an additive, binder or cobinder for different applications.
Composite particles comprise organic domains, for example an organic polymer matrix, and inorganic domains, for example fine inorganic solids, and typically have diameters of 4 to 5000 nm. The organic domains and the inorganic domains are generally joined to one another via physical or chemical bonds.
In commonly used processes for producing composite particles, organic monomers are polymerized by means of emulsion polymerization in aqueous dispersions of inorganic particles, with anchoring of organic domains on the surface of the inorganic particles, as described, for example, in Dong-Ming Qi, J. of Applied Polym. Sci., 2006, vol. 99, pages 3425 to 3432; Tsutomu Mizutani, J. of Applied Polym. Sci., 2006, vol. 99, pages 659 to 669; Frank Bauer, Macromol. Mater. Eng., 2006291, pages 493 to 498, or from DE-A 10 2004 010 155, U.S. Pat. No. 3,544,500, U.S. Pat. No. 4,421,660, WO-A 2006/072464 or WO-A 2010/118961.
However, the fixing of the organic and inorganic domains of the composite particles and the provision of stable composite particles present problems. This is because the inorganic particles or starting materials thereof and the organic monomers or the organic polymer matrices usually have different polarities and have a tendency to separate from one another or each to agglomerate with one another.
If such an agglomeration occurs before or during the production of the composite particles, for example, the agglomerated inorganic particles are encapsulated by the organic polymer matrix in the course of polymerization of the organic monomers, such that there is no homogeneous fixing of inorganic particles on the organic polymer matrix and hence ultimately no formation of chemically homogeneous composite particles formed from organic and inorganic domains. Corresponding mixtures in solvents do not take the form of colloidal primary particles. It may even be the case that the inorganic particles and the organic polymer matrix are present alongside one another as a blend.
However, in composite particles formed homogeneously from inorganic and organic domains too, there may be agglomeration of the inorganic or organic domains, which leads to inadequate storage stability, especially of dissolved, emulsified or dispersed composite particles, and is manifested by gellation or speck formation. Especially at relatively high temperatures, for example from 40° C., such an agglomeration occurs. Equally, aqueous dispersions of particles formed exclusively from inorganic units, for example colloidal silica or organopolysiloxanes, have a tendency to agglomeration at relatively high temperatures, for example at 70° C., or even at room temperature.
Agglomerated composite particles no longer have the desired performance properties or are even entirely unusable. In order to provide, for example, stable aqueous dispersions of composite particles, emulsifiers, protective colloids or specific additives are typically added as stabilizers, for example the hydroxyl group containing alkylamino compounds recommended in DE-A 10 2004 010 155.
To solve the aforementioned problems, WO-A 2009/112370 proposes composite particles which have been produced by condensation of siloxanes or polysiloxanes in the presence of polymers containing, for example, silane, amine or epoxy groups. However, aqueous dispersions of the composite particles thus obtainable with high solids contents have high viscosities, such that they can be handled only with difficulty. Finally, providing composite dispersions with high solids contents in stable form also presents particular difficulties. The aforementioned problems occur to a particular degree in the case of aqueous composite dispersions having solids contents above 30%.
It was an object of the present invention to provide processes for producing modified composite particles, with which the abovementioned disadvantages are avoided. More particularly, modified composite particles should thus be obtainable, these also being storage-stable in the form of dispersions with high solids contents and having a low viscosity.
The object was surprisingly achieved by fixing fine inorganic solids onto organic polymers, for example in the course of condensation of silicon compounds in the presence of organic polymers, and then polymerizing ethylenically unsaturated monomers in the presence of the composite particles thus obtained, forming the inventive modified composite particles.
There are known composite particles which are obtained by free-radically initiated copolymerization of ethylenically unsaturated organic monomers and ethylenically unsaturated inorganic particles, for example the ethylenically unsaturated siloxanes proposed in EP-A 1620271.
WO-A 2007/057382 describes silane-modified polyvinyl alcohols which are obtained by free-radically initiated polymerization of ethylenically unsaturated, silane-containing monomers in the presence of polyvinyl alcohols.
DE-A 102007038333 discloses compositions which have been produced by condensation of silicon compounds in the presence of polyvinyl alcohols, although the inorganic particles formed were not fixed on polyvinyl alcohols but were present alongside these in the composition as a blend.
EP-A 1243619 discloses composite materials consisting of organic domains, such as polyacrylic acid, and inorganic domains, for example sodium silicates or colloidal silica, the organic and inorganic domains being joined by ionic interactions with divalent metal cations, such that the composite materials are present as aggregates of the polymeric domains in the form of gels.